1. Field of the Invention
The present invention relates to a wireless microphone apparatus for performing frequency modulation (FM) of a voice signal for transmission as radio waves, and particularly to a transmitter device for FM wireless microphone with a band of about 800 MHz.
2. Description of the Related Art
Wireless microphone systems used in a concert halls, live stages or wayside recording are roughly classified into two types: (1) hand-held microphone transmitter systems and (2) belt pack microphone transmitter systems. Ultimately, the respective transmitter systems should be compact, lightweight and offer long term operation (about ten hours). However, it is difficult to achieve both size and weight reductions along with long-term operation because the transmitter systems are battery operated.
A hand-held transmitter apparatus typically has an integral structure of a microphone element, a window screen and a body case for receiving a battery. In addition, it is becoming desirable to be able to change certain components within a microphones in order to adapt it to differences in voice quantity, voice quality or preference of tone characteristics of a singer or a speaker.
Accordingly, it is desired to create a wireless microphone where the microphone is severable from the transmitter. However, this is complicated by the number of different circuits required to improve sound quality such as reducing signal-to-noise ratio or increasing dynamic range. Because of the large number of circuits, the body of the transmitter must be formed in a large size which is inconvenient and potentially interferes with the operation of the transmitter. Thus, conventional apparatus and circuits result in a microphone which is not optimal.
For example, in a conventional transmitter for a wireless microphone, a high-frequency oscillating part generally requires multiplying an output from a quartz oscillator to obtain a desired frequency. Such multiplication is done using a Phase Lock Loop (PLL) circuit and a conventional quartz multiplication method.
Such a conventional quartz multiplication method is illustrated in FIG. 7. The method involves multiplying a fundamental frequency of 23 MHz derived from a quartz crystal by 36 in order to obtain a desired frequency of 828 MHz related to a frequency band of 800 MHz. Such a multiplication method requires a circuit which consumes a significant physical mounting area. In part, the large surface area is due to intermediate filters required to reduce unnecessary radiation. In addition, in the case of an overtone oscillation type of the quartz oscillator, it is difficult if not impossible to perform deep modulation and oscillation with frequencies at or above 800 MHz. In part, this is due to the possibility of breaking the quartz oscillator which must be made very thin.
Alternatively, such conventional methods allow for frequencies that are easily changed. Such changes can be made by modifying circuit configuration. However, as illustrated in FIG. 8, such a circuit is complex. In addition to complexity, the circuit consumes significant power and thus requires larger or additional batteries. Thus, such conventional methods are not suited for miniature applications.
Accordingly, circuits and/or methods capable of miniaturization are desired. Further, it is desirable to provide a wireless microphone which provides for connection of a transmitter to a microphone through a standard connector, such as a canon connector (i.e. XLR).
The present invention provides a compact and lightweight wireless microphone for business. In addition, the present invention provides a transmitter for the microphone with a size compatible with a canon connector. In view of the aforementioned problems, it is an object of the present invention to provide a transmitter for a wireless microphone. The transmitter comprises a connector means for making connection to a microphone and inputting a voice signal from the microphone, a low-frequency amplification means for amplifying the voice signal, a surface acoustic wave (SAW) oscillation means for outputting a high-frequency oscillation signal, a modulation means for performing frequency modulation of the high-frequency oscillation signal by the amplified voice signal, a high-frequency amplification means for amplifying a high-frequency modulation signal, an antenna means for radiating a high-frequency output signal amplified by radio waves, and a power source for supplying electric power of a predetermined voltage to each of the aforementioned means.
To significantly improve sound quality required for business applications, the present transmitter device further comprises a compander circuit for improving a signal-to-noise ratio and for increasing a dynamic range by compressing the voice signal using logarithmic compression. In an embodiment, the compression is done using xc2xd logarithm compression. Beyond compression, a pre-emphasis circuit is included for reducing noise by providing a degree of amplification of high pass frequencies larger than the degree of amplification for other frequencies.
In some embodiments, the present invention comprises a SAW oscillation means which outputs a high-frequency oscillation signal with a band of about 800 MHz directly without multiplication.
The modulation means includes an oscillation element, a modulation and oscillation circuit, and a buffer amplification circuit. The voice signal is superimposed on a bias voltage at a variable capacitance diode at the modulation means. The voice signal is applied to the oscillation element and modulation is performed by changing a load of the SAW oscillation means. The SAW oscillation means is inserted into a terminal of the oscillation element.
The modulated signal is amplified using a high frequency amplification means. The amplified modulated signal has a power in an 800 MHz band of about +10 dBm. Additionally, an embodiment of the present invention provides for a band-pass filter circuit for eliminating harmonics of the high-frequency oscillation signal and for matching the impedance of the antenna means.
In part, the antenna means comprises the body case of the transmitter. The body case is about xc2xc the wavelength of the high-frequency oscillation signal. In an embodiment, the body case is about 7.5 cm long with a diameter of 2.3 cm. In an embodiment, the power source means comprises a battery and a DCxe2x80x94DC converter for increasing an output voltage of 1.5 V DC of said battery to 3.0 V DC. In some embodiments, the converted voltage is stabilized to 2.7 V DC by a series regulator. Each the circuit means in the device is designed to operate at the same voltage. In an embodiment, the battery used is a single AAA dry battery offering 10 hours or longer of continuous operation. In part because of the battery, a weight of the present device including the metal receiving case and the battery is about 55 g.
In some embodiments, the SAW resonator is capable of directly oscillating at or about a frequency band of 800 MHz. To avoid frequency changes of the SAW resonator due to changes in ambient temperature, an embodiment includes a plurality of transistors connected in series to increase negative resistance and thus provide sufficient oscillation margin.
Moreover, in some embodiments, a high-frequency amplifier performs matching for an antenna and a band-pass filter reduces unnecessary radiation. Such a band pass filter is simple because the SAW resonator directly provides a frequency at or about 800 MHz.
The antenna means provides good radiation efficiency. In an embodiment, such antenna means are formed from a microphone body while a transmitter body forms a ground. Accordingly, a structure in which the antenna is not exposed outside of the structure is achieved.